Single crystal of zirconia is transparent, and zirconia single crystal (cubic zirconia) containing about 10 mol % of yttria is conventionally utilized in jewelry goods and the like. On the other hand, it is known that general zirconia sintered body that is polycrystal is opaque. It is known as the cause that pores present between crystal grains and in the grains generate light scattering, and investigations that pores are decreased to impart transparency to polycrystal zirconia sintered body are made until now.
For example, yttria-containing zirconia sintered body having transparency is disclosed in U.S. Pat. No. 3,525,597 (TABLE III) (which is hereinafter referred to as “Patent Document 1”), and it is reported that in-line transmission of a zirconia sintered body containing 6 mol % of Y2O3 to a visible light is 11% at a thickness of 1.2 mm.
Light-transmitting zirconia containing 2 mol % or more of Y2O3 and from 3 to 20 mol % of TiO2 is disclosed in JP-A-62-91467 (Claims) (which is hereinafter referred to as “Patent Document 2”), but the presence of TiO2 is essential. (The term “JP-A” as used herein means an “unexamined published Japanese patent application”.)
Further, yttria-containing zirconia sintered body having transparency produced under ultrahigh pressure of from 500 to 3,000 MPa is reported in Journal of Less-Common Metals, vol. 13, p 530 (1967) (TABLE II) (which is hereinafter referred to as “Non-Patent Document 1”), and it is reported that in-line transmission of a zirconia sintered body containing 15 mol % of Y2O3 to a visible light is 10 to 14% at a thickness of 1 mm or less.
Light-transmitting zirconia disclosed in Patent Document 1 and Non-Patent Document 1 each has low in-line transmission, and although there is certain light transmission properties, it has been difficult to say to be transparent.
In general, in-line transmission can be represented by the equation (1), and is determined by scattering coefficient and sample thickness.ln(T)=βt−2 ln(1−R)  (1)
T: In-line transmission (%)
β: Scattering coefficient (mm−1)
t: Sample thickness (mm)
R: Reflectance R=((1−n)/(1+n))2 (n: Refractive index of zirconia)
(Refractive index of yttria-containing zirconia is about 2.2. Details are described in D. L. Wood, APPLIED OPTICS, vol. 29, No. 16, 2485-88 (1990))
For example, when the value (11%) at a sample thickness of 1.2 mm described in Patent Document 1 is converted to 1 mm using the equation (1), the in-line transmission corresponds to 16.6%, and it has been still difficult to say to be transparent.
It is exemplified that the light-transmitting zirconia disclosed in Patent Document 2 has in-line transmission of from 40 to 66% to a visible light of 600 nm at a thickness of 1 mm, and it can be considered to be a transparent sintered body to a certain extend. However, to exhibit such a transparency, it was described that it was essential to contain much TiO2 as 3 to 20 mol %. It is described in the Comparative Example that 8 mol % Y2O3-containing zirconia sintered body that does not contain TiO2 has in-line transmission of about 7% (thickness 1 mm). It is described that TiO2 promotes grain growth of a sintered body, and there was the problem on, for example, decrease of strength due to that grains become large. Further, as the characteristic that the yttria-containing zirconia possesses, the point that plasma resistance is excellent is exemplified, but there was the problem that the characteristic is impaired by containing TiO2 in large amount.
The present invention provides a stabilized zirconia polycrystalline sintered body that does not contain a transition metal oxide such as TiO2 or contains the same in an extremely reduced amount, uses yttria as a major stabilizer to zirconia, has high transparency and is chemically stable.